Antifreeze in heating system and leaving house for winter

My parents have a house on Cape Cod, MA, will be left empty for the winter. They close off the rooms with no plumbing and turn the heat in the main house down to the minimum. The heat is forced hot water with auto antifreeze.

I have a few questions. Should this antifreeze be changed and how often? What are possible problems to having antifreeze in a heating system? I feel it would be better to turn off the water in the house to prevent a possible flood and risk the boiler not working if the pressure gets to low to circulate the water. There might be some broken plumbing but at least the house won't be flooded.

I don't like the idea of auto antifreeze, it could contaminate their 'potable' water if makeup water has to be added to the hydronic heat system. They'd ought to use nontoxic antifreeze used for just this type application, along with potable water lines and toilets etc.. You find it at most plumbing supply houses and camping equipment dealers (if not Lowe's etc.).

If they were to drain and blow out the water lines, they wouldn't have to worry about them freezing. And I'd suggest doing that.